Hydrocarbons (oil, natural gas, etc.) are obtained from a subterranean geologic formation (i.e., a “reservoir”) by drilling a wellbore that penetrates the hydrocarbon-bearing formation. In order for the hydrocarbons to be produced, that is, travel from the formation to the wellbore, and ultimately to the surface, at rates of flow sufficient to justify their recovery, a sufficiently unimpeded flowpath from the subterranean formation to the wellbore, and then to the surface, must exist or be provided.
A common reason for a decline in hydrocarbon production is the formation of scale in or on the wellbore and in the near-wellbore area or region of the hydrocarbon-bearing formation matrix. Generally, scale formation occurs from the precipitation of minerals, such as barium sulfate, calcium sulfate, and calcium carbonate, which become affixed to or lodged in the wellbore (or wellbore casing) and/or the near-wellbore matrix. Scale is generally produced from dissolved minerals carried in water which is unintentionally produced along with the desired hydrocarbon. When the water (and hence the dissolved minerals) contacts the wellbore or the near wellbore region, often accompanied by a significant change in pressure and/or temperature from its source in the formation, the dissolved minerals may begin to precipitate, forming scale. Not uncommonly, scale may form to such an extent that it can completely choke off a wellbore. In addition, the scale formed is often very hard, so that mechanical techniques for its removal, e.g., milling, are undesirable since they often damage the steel casing lining the wellbore. Prior art chemical methods of scale removal, such as injection of a solvent into the wellbore to contact the scale, are often ineffective against certain types of scale, particularly barium sulfate (BaSO4) scale. Generally speaking, current commercial state-of-the art systems utilize ethylene diamine tetraacetic acid (EDTA). Numerous other systems have been proposed, studied, patented, and even field tested, but none of these systems has enjoyed widespread commercial success. Prior to the invention, therefore, no satisfactory or universally accepted solution has existed for the removal of BaSO4 scale from a wellbore, or for the removal of BaSO4 in combination with other types of scale.